A patient recovering from heart surgery or suffering a myocardial infarction must be kept under constant observation until his heart condition improves. Monitoring the electrocardiac signals, sometimes called ECG signals, produced by the expansions and contractions of the patient's heart is a common method of observation during this time. These electrocardiac signals are present on the skin and throughout the body. They are a valuable medical indicator because their shape and repetition rate can indicate to a trained observer whether the heart is operating properly or nearing a dangerous condition.
During the initial phase of a heart patient's recovery, he is bedridden and directly connected to a bedside monitor, such as an electrocardiograph. The monitor is usually wired to electrodes that are attached to the skin near the heart. The electrodes detect the electrocardiac signals that are circulating on the skin, and the wires transmit them to the monitor.
When a patient's condition improves, it is often desirable to let him move about. This is difficult if he remains connected to the bedside monitor because the wires restrict his movement. To remedy this problem, a telemetry system is sometimes used to replace the direct wired connection.
The telemetry system includes a portable transmitter carried by the ambulatory patient and a stationary receiver connected to the monitor. Electrodes still sense the electrocardiac signals, but now the signals are transmitted by radio waves to the receiver. At the receiver, the transmitted signal is demodulated and the resultant electrocardiac signal is conveyed to the monitor. With such a telemetry system, a heart patient can move about while his electrocardiac signals are kept under constant surveillance.
If a heart monitoring system becomes inoperative, a special indication should be given to the monitor operator so that the fault can be quickly corrected and so that the inoperative condition will not result in confusion and create a false heart rate alarm. Because of the increased movement of an ambulatory patient, there are more problems involved with a telemetry monitoring system than with a stationary monitor. Patient movement may disconnect an electrode, stopping detection of the electrocardiac signals, or it may shift the position of an electrode, weakening the detected ECG signals. An ambulatory patient may also move out of the range of the receiver and ruin the reception.